[Mesa-dev] S2TC - yet another attempt to solve the "S3TC issue"

[Dan McCabe]

On 08/03/2011 12:47 PM, Ian Romanick wrote:
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> On 08/03/2011 12:11 PM, Bryan Cain wrote:
>> On 08/03/2011 01:58 PM, Ian Romanick wrote:
>>> I think this solves the issue for the compressor and for the software
>>> decompressor.  I don't think this solves the problem for the
>>> decompressor for hardware drivers, so that may still pose a problem for
>>> Linux distros.
>> Pardon my ignorance, but why do hardware drivers need a decompressor?
> To quote the EXT_texture_compression_s3tc spec:
>
>      WARNING:  Vendors able to support S3TC texture compression in Direct3D
>      drivers do not necessarily have the right to use the same
> functionality in
>      OpenGL.
>
> This is the same issue that Linux distros have with ARB_texture_float
> being enabled in hardware drivers.  The hardware may implement the
> functionality, and the hardware vendor may have some license for the
> patent, but that license may not cover making the functionality
> available in Mesa.  That S3 has sued Apple is some indication that these
> licenses may have very narrow scope.

Not being a lawyer (nor even playing one on television), all this is 
doing is feeding data in a form the hardware can interpret. The 
algorithms used in the creation of the data in this case bears no 
resemblance to any algorithms  that create "high quality" data that the 
hardware, which was created under license from S3, might consume.

You could argue that using the next courser MIP level (2x2 subsampling 
with low-pass filtering) in an RGB565 format with dithering to get a 
virtual 32-bit color space accomplishes a similar effect with a 
comparable decrease in the memory footprint, since S3TC uses 2 16-bit 
colors and a 4x4 matrix of 2-bit fields per 4x4 collection of texels. 
This shouldn't infringe on S3's S3TC compression and/or decompression 
algorithms (although I cannot comment on whether it infringes on any 
other patents).

YMMV. If you want a definitive answer, you might want to consult an 
attorney who is willing to defend you in the event of infringement. 
Otherwise, all bets are off. But if I were a gambling man (and weren't 
employed by a company that may not want to place such a bet without 
appropriate legal advice), ...

Ultimately, this a legal, not a technological, question.

cheers, danm

Disclaimer: I wrote the original reference implementation of S3TC as 
well as the Game Developer article with John Brothers that described 
S3's technology while an employee of S3. I no longer work for S3. Also, 
I am not a lawyer (see my first point in the reply).


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